The considerable growth of the polyurethane-producing industry has been accompanied by the problem of removing and re-using polyurethane waste or damaged products. A market has been found for polyurethane flexible foam scrap material by bonding this material to form composite bodies, but only a limited amount of waste flexible foam materials may be used in this manner. No similar application exists for semi-rigid and rigid polyurethane foam waste or for elastomer granulates. Therefore, large quantities of rejected and damaged polyurethane products resulting from the production of rigid and flexible foams and of elastomers have to be deposited on dumps or destroyed in a refuse incinerator. These methods involve serious ecological, technical and economic problems due to the low specific weight and to the associated large volume of the rejected or damaged goods.
Therefore, for ecological and economic reasons, there is a considerable interest in economically recycling the constantly-increasing quantities of polyurethane waste. Several processes are known for the working-up and/or for the degradation of polyurethane plastics waste by glycolytic cleavage, in some cases with the addition of amino alcohols and/or catalysts. Such processes are known, for example, from German Auslegeschriften or Offenlegungsschriften Nos. 1,110,405; 2,238,109; 2,304,444; 2,414,091; 2,516,863; 2,557,172; 2,738,572; 2,759,054 and 2,902,509; from U.S. Pat. Nos. 3,632,530; 4,014,809; 4,110,266; 4,159,972 and 4,162,995; and from Japanese Patent Nos. 51 006-909; 52 004-596; 53 022-595 and 56 099-244.
Many of these publications clearly indicate that none of these known processes provides a satisfactory solution to the problem. The commercial scope of application and the economy of the described processes are actually severely restricted by a number of disadvantages:
(1) The dissolution and reaction times, which are generally several hours, result in unsatisfactory volume-time yields, which cause economic problems.
(2) The quantities of diol required for the dissolution and degradation of the polyurethane waste are within the range of equal weight quantities, but are often higher, so that the resulting reclaimed polyols quantitatively amount to several times the amount of original waste and, additionally, in many cases, more reactive and more expensive amino alcohols are used simultaneously with the diols in order to affect the degradation of the wastes.
(3) For the long reaction times which have been described (for example, from 2 to 12 hours), the degradation temperatures of up to 250.degree. C. result in undesirable secondary reactions and in a thermal impairment to the recovered polyols, limiting the amount of these reclaimed polyols which may be blended with a pure polyol. With respect to the quantities of regenerated polyol which clearly exceed the quantities of original waste, this limitation results in very great difficulties when returning these recovered polyols into the production process.
(4) A limited curtailment of the reaction time and/or a reduction in the degradation temperature may be achieved by shifting the pH, by adding certain catalysts and/or by the simultaneous use of codegraders, for example ammonia, amines or alkanolamines, but the properties of the recovered polyols which are changed by the catalysts or codegraders have a disadvantageous effect on the re-use of these polyols.
(5) Polyurethane plastics are characterized by the extraordinarily-varied chemical structure thereof. Depending on the desired properties in each case, in addition to urethane bonds, they may also contain urea, biuret, allophanate, isocyanurate, carbodiimide and/or ester groups. This requires an optimization of the degradation conditions which is related to the formulation in each case, and a specific selection of suitable degradation diols or diol-co-degrader mixtures. The claims of the above-mentioned patents which are very specific in some respects are also to be understood within this sense. Thus, for example, German Offenlegungsschrift No. 2,304,444 specifically claims the degradation of polyisocyanurate waste, while Offenlegungsschrift No. 2,414,091 provides the degradation of polyurethanes containing carbodiimide groups. Therefore, polyurethane waste mixtures of varying compositions and a waste of an unknown formulation cannot be worked-up in a commercially-satisfactory manner by any single known process.
(6) The number of materials which are flame-retarded by the addition of chloroalkyl phosphates is increasing in the field of flexible and rigid polyurethane foams. In the glycolytic cleavage of foam waste, phosphate esters of this type produce recovered polyols which have high acid numbers, which do not allow a direct use, and which necessitate an after-treatment with propylene oxide (Kunststoff-Journal No. 5 (1975), pp. 24; Polymer Eng. Sci. 18 (1978), No. 11, pp. 846; SYSpur Rep. 1977, Part 12, pp. 56-65). However, working with propylene oxide requires a considerable expense in apparatus and with regard to safety.
Surprisingly, it has now been found that polyurethanes of the most varied compositions may be degraded into reusable polyols in an economic, controllable and continuous manner. This process may be completed in a short time and at an elevated temperature, with a relatively low requirement of degradation glycol, without substantially impairing the recovered polyols, and without the above-mentioned disadvantages, in a widely-applicable method using a specifically equipped screw machine. Hitherto, the use of screw machines has only been described for the irreversible, continuous hydrolysis of polyurethane waste, as in German Offenlegungsschrift No. 2,442,387. It came as a complete surprise that machines of this type may also be used in an economic manner for continuously carrying out reversible transurethanization reactions, because glycolysis is essentially an equilibrium of transurethenization, while hydrolysis is an irreversible reaction with the release of CO.sub.2.